Many types of spectroscopies require that a sample to characterize be illuminated with light having two different wavelengths simultaneously. Then, the interaction of the sample with this simultaneous illumination is characterized.
Characterization of optical nonlinearities in substances is of great importance for many applications. For example, in Stimulated Raman Scattering (SRS), a substance is illuminated with pulses of laser light having two different wavelengths. Laser light at a first wavelength is pulsed at a first pulse frequency. Laser light at a second wavelength, that interacts with the laser light of first wavelength in the substance to produce the SRS effect, is pulsed at a second pulse frequency, which is typically substantially half the first pulse frequency. The first and second laser lights are synchronized such that at predetermined time intervals, the substance is simultaneously illuminated with the first and second laser lights, and at other time intervals, only one of the first and second laser lights illuminates the substance. This same method is usable to perform Coherent Anti-stokes Raman Scattering (CARS) measurements.
A disadvantage of this method resides in that when the nonlinear effect is present, in other words when the substance is illuminated with first and second laser lights, the optical power received by the substance is different from the power received when only one of the laser lights is present. Therefore, it is relatively difficult to quantitatively assess the non-linear interactive effects because of these power variations.
Also, the acquisition of a spectrum requires that the wavelength of one of the first and second laser lights be varied in time. This process is typically relatively slow.
Against this background, there exists a need in the industry to provide an improved mode-locked laser. An object of the present invention is therefore to provide such a device.